var stream = require('stream')
var Readable = stream.Readable
var rs = new Readable()
var c = 'a'.charCodeAt(0)
rs._read=function(){
    if(c<='z'.charCodeAt(0)){
        rs.push(String.fromCharCode(c++))
    }else{
        rs.push(null)
    }
}
//将可读流输出到一个可写流上
rs.pipe(process.stdout)